Laser light is utilized as a heat source, a light source, and the like, and also generally used as a light source for image recording.
Recording systems using a laser as a light source include a scanner system image formation method in which an original is scanned and a light-sensitive material is exposed to light according to the resulting image signals, and a recording system in which information is recorded on an optical recording medium called a photo disc.
Semiconductor lasers oscillating infrared light have been applied to these recording systems to reduce the size of equipment. However, use of infrared light has limited the stability or recording density of light-sensitive materials. In order to solve this problem, a semiconductor laser is desired which oscillates in a wavelength region shorter than the visible region, but this has been difficult to realize. On the other hand, much attention has recently been directed to materials having nonlinear optical effects, which, when irradiated with a strong laser beam, provide transmitted light having different components from incident light through the mutual interaction therebetween. Such materials are generally known as nonlinear optical materials and are described in detail, e.g., in David J. Williams (ed.), Nonlinear Optical Properties of Organic and Polymeric Materials, ACS Synposium Series 233, American Chemical Society (1983), M. Kato & H. Nakanishi (eds.), Organic Non-linear Optical Materials, CMC Co. (1985), D. S. Chemla & J. Zyss (eds.), Nonlinear Optical Properties of Organic Molecules and Crystals, Vols. 1 and 2, Academic Press, 1987, etc. It has been proposed, instead, to combine a semiconductor laser oscillating infrared light with a wavelength converter element composed of a nonlinear optical material to thereby obtain a laser beam in the visible region. However, nonlinear optical elements containing inorganic compounds, e.g., LiNbO.sub.3, KH.sub.2 PO.sub.4, LiIO.sub.3, BaB.sub.2 O.sub.4, etc., exhibit low efficiency in wavelength conversion. On the other hand, conventional organic compounds with high efficiency, such as 2-methyl-4-nitroaniline (MNA), 2-N,N-dimethylamino-5 -nitroacetanilide (DAN), m-nitroaniline (m-NA), L-N-(4-nitrophenyl)-2-(hydroxymethyl)pyrrolidone, etc., have low transmittances of blue light due to their strong yellow color, failing to convert incident light to sufficiently shorter wavelength light.